Ceramic fibers prepared from various organosilicon polymers are well known in the art. These fibers have a broad array of utilities such as reinforcement for plastic, ceramic and metal matrices to produce high performance composite materials or the formation of fibrous products such as high temperature insulation, belting, gaskets and curtains.
Unfortunately, the room temperature tensile strength of these fibers diminishes rapidly after being exposed to temperatures of 1200.degree. C. and higher for longer than 30 minutes. End products produced from these fibers, therefore, are also expected to have only marginal service life in this environment.
Early explanations for the decreased tensile strength after high temperature aging primarily centered around the evolution of gaseous species. Mah et al., for instance, in J. Mat. Sci. 19 (1984) 1191-1201 described the degradation behavior of Nicalon.TM. (Si-C-O fibers, Nippon Carbon Co.) fibers after heat treatment. They discovered that fiber strength degraded at temperatures in excess of 1200.degree. C. regardless of heat treatment conditions. They also discovered that degradation was often associated with evaporation of CO as well as beta-SiC grain growth. No solutions to this problem were suggested.
Luthra in J. Am. Ceram. Soc., 69 (10) c-231-233 (1986) examined the inherent instability of Si-C-N-O series ceramic fibers at elevated temperatures (Above 1000.degree. C.) and concluded that the fiber composition is expected to change (by gas evolution), irrespective of the environment used for pyrolysis. The possible solutions suggested therein to overcome the above problems included eliminating oxygen and nitrogen from the polymer or replacing the free carbon in the ceramic with free silicon.
Lipowitz et al. in Advanced Ceramic Materials, Vol. 2, No. 2, 121 (1987), which is incorporated herein in its entirety, describes the composition and structure of 4 melt spun Si-C-N-O series fibers including standard grade Nicalon.TM., ceramic grade Nicalon.TM., fibers derived from methylpolydisilylazane and fibers derived from hydridopolysilazane. It is disclosed therein that temperatures at or above 1300.degree. C. cause degradation of these fibers through the loss of gases such as CO, SiO and N.sub.2 and through the formation of coarse beta SiC grains.
Johnson et al. in J. Am. Ceram. Soc., 71 (3) C-132-C-135 (1988) discuss the thermal degradation mechanisms of Nicalon.TM. and a silicon carbide fiber deposited by chemical vapor deposition (CVD). They suggest therein that the loss in strength of CVD fibers is largely a result of the loss of CO whereas the loss in strength of Nicalon.TM. fiber results from the loss of structurally bound Si as SiO. The authors disclose that the localization of calcium on the surface of CVD fibers may be potentially deleterious in terms of fiber degradation but that it is not known whether the transport of calcium to the surface of other types of SiC fibers significantly affects their strength. This reference, therefore, does not disclose the effect of metal or metal compound impurities on polymer-derived ceramic fibers.
The present applicants have now discovered that heterogenously distributed impurities such as metals or metallic compounds cause the high temperature instability observed in polymer derived ceramic fibers of the Si-C-N-O series. Moreover, removal of said impurities has led to products with unique high temperature stability and, thus, will extend high temperature service life of composites made therefrom.